This invention relates to pulse position modulation (PPM) of data for transmission over a channel, such as a fiber optic channel, and more particularly to a method and apparatus for synchronizing a receiver directly from the PPM stream without the use of decision aided feedback.
The PPM format for transmission of data is well suited for optical communication channels. In the PPM format, an interval is divided into a number, M, of slots or positions where a pulse of duration .tau..sub.p seconds may occur. For example, if M is equal to 256, each word represents 8-bits of information. Therefore, the transmission of binary valued PPM pulses is a very efficient method of transmitting data with optical pulses that can be generated at high rates, typically in the megahertz range.
At the receiver, a photomultiplier tube is used to detect the weak PPM optical pulses. The problem is that since pulse position in the M-ary PPM data stream is critical, it is necessary to synchronize the receiver and track the weak optical PPM signal. Heretofore there has been no method known for establishing pulse position (slot) synchronization directly from a detected optical M-ary PPM data stream without the use of decision aided feedback, and without significantly altering the performance (error probability or information rate) of the optical receiver.